1. Field of the Invention
The present invention relates to an interferometer for use in measurement of the shape of a measuring object based on interference fringes, and a shape measuring method using the interferometer.
2. Description of the Related Art
An interferometer for use in measurement of flatness generally incorporates a high-coherence light source such as a He—Ne laser. On measurement of a parallel glass substrate, however, a light reflected from a front surface to be measured (a target surface) and a light reflected from an undesired, rear surface may interfere with each other, resulting in interference noises. The use of a low-coherence light source may prevent a rear-surface reflection from causing interference noises if a coherent length ΔL is determined sufficiently shorter than an optical path length d of light traveling between the front/rear surfaces (see JP-A 2001-91223).
On the other hand, a coherent length ΔL determined too shorter is not available in measurement unless a difference between an optical path length of a measurement light and an optical path length of a reference light (an optical path difference) is determined less than the coherent length ΔL. Therefore, this makes it difficult to measure a target surface with large waviness and requires strict efforts by a skilled worker on setting of a measuring object.